Traditionally, the speed (revolutions per minute) and torque (horsepower) of a DC motor have been controlled by varying the magnitude of the voltage applied across the motor and/or the magnitude of the current passed through the motor. One technique employed a variable resistance connected in series with the DC power source and the windings of the DC motor. Another technique employed a variable shunt resistance connected across the motor terminals in parallel with the motor. A third technique employed a combination of series and parallel connected variable resistances. In each of these techniques, a substantial amount of the power provided by the DC power source is wasted due to the power loss in the variable resistance throughout the normal range of motor speed. The dissipation of power in the variable resistance also results in the production of heat in the control circuit, which can be detrimental.
A fourth technique avoided the use of the variable resistance elements by employing an interrupter or chopper to produce "on" pulses having a variable width in order to control the average current available to the motor by changing the pulsewidth of the "on" pulses. However, when a DC voltage is applied to or removed from a DC motor, the inductive components of the motor react to produce voltage and current excursions within the interconnected power system. This inductive reactance is opposite in polarity with the desired change in DC current and detracts from the motor performance. In order to minimize this effect, diodes have been placed across the motor terminals to absorb the inverse voltage spikes. As a result of the diode action, the motor can temporarily overspeed or "freewheel" instead of resisting changes in the RPM of the motor.
The inductive components of the DC motor are inherently sensitive to frequency. The pulsing rate (frequency) used in controlling a DC motor must be high enough to overcome the tendency of the motor to "buck" or "jump" on startup or when running under load at low RPM. However, if the motor RPM is above the stall range, the pulsing rate can be reduced significantly.
Some solid state motor control devices, e.g. the system of Nelson, U.S. Pat. No. 5,029,229, have utilized a voltage signal from a speed control lever as an input to a high frequency generator to vary the on-off ratio of fixed frequency pulses which are supplied to a power FET circuit to control the power flow to a DC motor. Some of these devices also employ freewheeling diodes to minimize the inverse voltage spikes which occur at the beginning and end of each pulse. These devices generally provide a DC power efficiency which is 10% to 20% better than that achieved with resistive controls. However, each on-off cycle of the high frequency pulses results in a power loss due to heat produced by the transition of the power FET devices from a high resistance value to a low resistance value.
Thus, there is still a need for a control circuit which can achieve a greater efficiency in the operation of a variable speed DC motor.